The IUPAC Compendium of Chemical Terminology defines an airlift bioreactor as “a bioreactor in which the reaction medium is kept mixed and gassed by introduction of air or another gas (mixture) at the base of a column-like reactor equipped either with a draught tube or another device (e.g. external tube) by which the reactor volume is separated into a gassed and un-gassed region thus generating a vertically circulating flow.” The definition refers to internal and external loop airlift bioreactors, although the present invention is concerned only with the internal-loop airlift bioreactor.
The majority of airlift bioreactors are traditionally made out of stainless steel and designed with pressure vessel criteria in mind to sustain the required steam pressure during sterilisation. The draft tube is also made in stainless steel, is cylindrically shaped and supported in a vertical position by arms connected to the inside of the bioreactor vessel. The air inlet is generally positioned just below the draft tube, so that the gas-liquid mixture rises inside the draft tube, although the opposite circulation mode is also possible when the air inlet is positioned outside the draught tube. Hence, the overall simplicity of the bioreactor and its geometrical relationship with the draft tube makes it possible to scale-up laboratory sized bioreactors without major difficulties to sizes of up to 10,000 liters.
A disposable airlift bioreactor suited to replace the traditional airlift bioreactor would be especially attractive since it could provide considerable reduction in capital investment as well as operating costs. The reason for this is the replacement of the stainless steel bioreactor vessel with a pre-sterilised plastic bioreactor container or bag which would only be used once; making the otherwise required sterilisation (e.g. steam) and cleaning process obsolete.
So far, several attempts have been made on disposable airlift bioreactors:
P. Whitney's air lift fermenter as part of Patent GB2202549 consists of a flexible daft tube connected at the top and bottom of the plastic bag whereby the air inlet is positioned below the daft tube. The problem with this approach is that suction forces within the draft tube cause it to collapse, preventing entry of subsequent air bubbles.
The invention ‘Air lift fermentor formed from flexible plastic sheets’ as described in patent EP0343885 uses a ‘divider panel formed of plastic film’ to divide the plastic bag into two regions and thus creating circulation in the bag with an air inlet device in one of the regions. In this approach there is no risk of shear forces inside the draft region causing the dividing panel to collapse since when the bag is filed, the panel is under tension and taut. To achieve this configuration, the two plastic films forming the bag need to be bigger in size than the divider film. This precondition presents difficulties during fabrication of the bag and requires additional folding to reduce the overall volume for sterilisation and shipping.
A disposable air lift bioreactor is describes as part of U.S. Pat. No. 6,432,698 whereby again a partition is used to create a draft tube. Here the partition can either be a divider inserted in the bag or can be created by fusing a short section of the two sides of the plastic bag together. This approach is suitable for bags of small volumes, but might not withstand hydrostatic pressures if applied to larger volumes.
Patent WO2005118771 titled ‘Disposable bioreactor systems and methods’ mentions the possibility of a divider inside the bag, more specifically ‘a sheet of plastic bisecting the chamber of the bag vertically’ to enable circulation with a gas inlet element. This concept however was already described in earlier patents (see above) such as EP0343885 and U.S. Pat. No. 6,432,698
In summary, the two approaches until now for a disposable airlift bioreactor have been based either on a flexible draft tube or on a divider panel. So far, both of them have not been suitable to replace the existing traditional airlift bioreactor. The draft tube in particular has represented a problem in translating the traditional airlift bioreactor to a disposable version since bags made out of plastic sheets were already used to store and transport liquids. Hence, the challenging aspect for a disposable airlift bioreactor being able to replace traditional bioreactors consists of a draft tube or other device to provide vertical circulation in the bag.
Nowadays disposable (plastic) bags are increasingly used to store and transfer solutions required for bioprocessing. The bags can be supplied pre-sterilised and their shapes and configurations vary extensively. The majority of theses bags are generally referred to as either two or three dimensional (2D & 3D), whereby a 2D bag consists of 2 sheets of plastic sealed at their perimeter while a 3D bag consists mainly of 4 sheets of plastic sealed and cut in a particular form to create a bag with 4 sides. There are of course other bags available.
It is therefore a feature of the present invention to provide a liquid circulation apparatus, and in particular a disposable airlift bioreactor which overcomes the above mentioned problems of the prior art.